Influence of geology and topography on the occurrence of mine massive collapse—back-analysis of a historical collapse of chalk mine (France)

Abstract

Pillars of an underground mine are designed to ensure long-term stability and preserve the surface assets from the consequences of mining. However, long-term stability is never guaranteed. Over time, subsidence and collapses can occur, and the consequences are sometimes dramatic. Such collapses depend on the exploitation characteristics, the mechanical properties of the terrain, and the potential presence of water, but additional factors can play a major role. This study aims to identify the predisposing and triggering factors of the collapse, including stiff beds, cliffs, and faults. Back-analysis was carried out on a historic dramatic massive collapse of a chalk mine (Beaulieu, France, 1910). Numerical modeling (finite and distinct elements) was carried out to highlight the influence of mining geometry, the stiff bed, and the cliff. Then, the mechanism of the collapse of the Beaulieu mine was investigated. The results demonstrate the importance of the geometric configurations of the site for the development of such a phenomenon, particularly the cumulative effects of the excavation (extraction ratio), the cliff, and the stiff bed. These cumulative factors increase the risk of a massive collapse. This study has shown that the stability of abandoned room and pillar underground mines should cover three concepts: estimation of the stress state of the pillars, the strength of the pillars, and the presence of a cliff as well as a significant discontinuity crossing the rock mass which could be a fault.